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1.
Longitudinal valleys form first order topographic features in many mountain belts. They are commonly located along faults that separate tectonic zones with varying uplift histories. The Indus Valley of Ladakh, northern India, runs northwestwards following the boundary between the relatively undeformed Ladakh Batholith to the north–east and the folded and thrusted Zanskar mountains to the south–west. In this region the Shyok Valley, on the northern side of the batholith, approximately parallels the course of the Indus. This study investigates geomorphic variations in transverse catchments that drain the Ladakh Batholith, into the Indus and Shyok rivers. The batholith has been divided into three zones based on varying structural characteristics of its northeastern and southwestern boundaries. Morphometric analysis of 62 catchments that drain into the Indus and Shyok valleys was carried out using three digital datasets, and supported by field observations. Morphometric asymmetry is evident in the central zone where the Shyok valley is considered tectonically inactive, but the Indus Valley is bound by the northeastwardly thrusting Indus Molasse and the batholith. In this zone the catchments that drain into the Indus Valley are more numerous, shorter, thinner and have lower hypsometric integrals than those that drain into the Shyok. By linking these observations with the regional geology and thermochronological data it is proposed that high sediment discharge from the deformed Indus Molasse Indus Valley has progressively raised base levels in the Indus Valley and resulted in sediment blanketing of the opposing tectonically quiescent catchments that drain southwestwards off the batholith. The Indus Molasse thrust front has propagated at least 36 km towards the Ladakh Batholith over the last 20 Ma. Hence it is proposed that this long term asymmetric structural deformation and exhumation has forced the Indus longitudinal valley laterally into the Ladakh Batholith resulting in the morphometric asymmetry of its transverse catchments.  相似文献   

2.
The 3-D distribution of the b value of the frequency–magnitude distribution is analysed in the seismically active parts of the crust near Long Valley Caldera, California. The seismicity is sampled in spherical volumes, containing N =150 earthquakes and centred at nodes of a grid separated by 0.3  km. Significant variations in the b value are detected, with b ranging from b ≈0.6 to b ≈2.0. High b -value volumes are located near the resurgent dome, and at depths below 5  km at Mammoth Mountain. b values are found to be much lower south of the Long Valley Caldera. We interpret this to indicate that an active magma body has advanced from depths below 8  km to depths of 4 to 5  km beneath Mammoth Mountain in 1989, and that anomalous crust, either highly fractured or containing unusually high pore pressure, such as is the case in the vicinity of active magma bodies, exists north of the seismically active area beneath the resurgent dome at all depths. We also investigate the spatial distribution of temporal variations of the frequency–magnitude distribution by introducing differential b -value maps. b values increased from b ≈0.8 to b ≈1.5 underneath Mammoth Mountain at the onset of the 1989 earthquake swarm and remained high thereafter. This suggests that an intrusion permanently altered the average distribution of cracks at 5–10  km depth, or that the pore pressure permanently increased. We propose that high b values are a necessary (but not sufficient) condition near a magmatic body, and therefore spatial b -value mapping can be used to aid in the identification of active magma bodies.  相似文献   

3.
The syntectonic continental conglomerates of the South‐Central Pyrenees record the late stages of thin‐skinned transport of the South‐Pyrenean Central Units and the onset of exhumation of the Pyrenean Axial Zone (AZ) in the core of the orogen. New magnetostratigraphic data of these syntectonic continental conglomerates have established their age as Late Lutetian to Late Oligocene. The data reveal that these materials were deposited during intense periods of tectonic activity of the Pyrenean chain and not during the cessation of the deformation as considered previously. The magnetostratigraphic ages have been combined with new detrital apatite fission track (AFT) thermochronology from AZ‐derived granite cobbles within the syntectonic conglomerates. Distribution of the granitic cobbles with different AFT ages and track lengths combined with their depositional ages reveal information on the timing and rate of episodes of exhumation in the orogen. Some AFT ages are considerably older than the AFT ages of the outcropping AZ granitic massifs, indicating erosion from higher crustal levels within the massifs than presently exposed or from completely eroded plutons. Inverse thermal modelling reveals two well‐defined periods of rapid cooling in the hinterland at ca. 50–40 and ca. 30–25 Ma, with another poorly defined cooling episode at ca. 70–60 Ma. The lowest stratigraphic samples experienced postburial annealing caused by the deposition of younger syntectonic sediments during progressive burial of the south Pyrenean thrust and fold belt. Moreover, samples from the deeper stratigraphic levels also reveal postorogenic cooling during the Late Miocene as a response to the excavation of the Ebro River towards the Mediterranean Sea. Our data strongly support previous ideas about the burial of the South Pyrenean fold and thrust belt by Late Palaeogene syntectonic conglomerates and their subsequent re‐excavation and are consistent with other thermochronological data and thermal modelling from the interior part of the orogen.  相似文献   

4.
Magnetic history of a dyke on Mount Etna (Sicily)   总被引：1，自引：0，他引：1  
During the 1989 eruption of Mount Etna, two fracture systems, trending c. N45°E and N150°E, opened at the foot of its 3000 m high SE Crater and propagated quickly downslope to distances of ≈3 and 7 km, respectively. The northeastern fracture fed a flank eruption, whereas the southeastern fracture remained dry and offered contrasting volcanological and geophysical evidence of the presence of magma at a shallow depth. During the opening of this non-eruptive fracture system, a differential magnetic network was set up on a short profile across its distant extremity. Initially, the magnetic field did not display any change along the profile between frequent surveys. However, repeated measurements at intervals of about 3 months for two years revealed the slow build-up of a 130 nT anomaly. The anomaly vanishes laterally within 0.2 km of the surface expression of the fracture system. This exceptional set of observations constrains the location and time of cooling of a shallow dyke. The increase in magnetization of the dyke inferred by the rate of growth of the anomaly leads to the interpretation that the dyke was emplaced near the end of the eruption.  相似文献   

5.
Summary. LITHOPROBE has acquired nearly 270 km of crustal seismic reflection data across the eastern portion of the southern Canadian Cordillera, These reflection profiles, obtained during the Fall of 1985, extend from the Rocky Mountain thrust and fold belt, across the Rocky Mountain Trench, Purcell anticlinorium, Kootenay Arc, Nelson batholith and Valhalla gneiss complex. North American basement and its overlying foreshortened miogeoclinal rocks can be traced westward to the Kootenay Arc. The Purcell anticlinorium is carried by a series of west dipping thrust faults which emerge east of the anticlinorium and converge downward and merge with a detachment surface above autochthonous North American basement. Proterozoic supracrustal rocks, thickened by folding and thrusting, occupy the core of the anticlinorium. Steeply dipping surface structures of the western Purcell anticlinorium and Kootenay Arc appear to be truncated at 3 - 4 s (9-12 km) by a gently east-dipping reflection that may delineate the upper boundary of an allochthonous wedge inserted between the near surface rocks and autochthonous basement below. Beneath the Kootenay Arc, at a travel time of 9–10 s (27–30 km), the North American basement seems to be truncated by the major east-dipping Slocan Lake fault zone, which can be traced from its surface exposure at the east edge of the Valhalla gneiss complex eastward to near the base of the crust. A high amplitude, west-dipping reflection underlies the Valhalla complex and may be related to a major compressional shear zone.  相似文献   

6.
The Wuyo-Gubrunde Horst in the northeastern Nigeria consists of migmatite gneiss, unaltered, altered, and sheared porphyritic granites, pegmatites, aplites, basalts, and sandstone. Uranium has been reported in rhyolite, sheared rocks, and sandstone within the area. The petrogenesis of the granitoids and associated rocks in the area was evaluated in the light of new geochemical data, which showed that the U content of altered porphyritic granite is highest and hydrothermal-related. The granitoids are metaluminous, sub-alkaline, and S-type granite, and have evolved by partial melting of crustal material emplaced at moderate depth of 20–30 km in a syn-to late-collisional within-plate tectonic setting. The negative Eu/Eu* anomaly and high (La/Yb)N ratio of the granitoids indicate magma fractionation. The low SiO2 (<53%) and high Fe2O3 (10%) of the altered porphyritic granite compared to other similar rock units suggest pervasive alteration. The associated basalts are tholeiitic, emplaced within continental plate tectonic setting, and enriched in Ni, V, Nb, Sr, and light rare earth elements, and they have SiO2, Fe2O3, V, Th, and Co contents that are similar to those of the altered porphyritic granites. The U occurrence in the Wuyo-Gubrunde Horst is believed to be sourced from the adjoining Bima sandstone in the Benue Trough, which locally contains carbonaceous zones with anomalously high concentrations of U. The Fe2+/Fe3+ redox fronts formed by alteration of the iron-rich basalts provided the requisite geochemical barrier for U-bearing hydrothermal fluid, causing enrichment of U leached and mobilized from the sandstone through fractures in the rocks.  相似文献   

7.
乔治王岛北海岸的第三纪火山岩包括菲尔德斯半岛组（ＦＰＧ）熔岩、Ａｄｍｉｒａｌｔｙ湾组（ＡＢＧ）岩颈和Ｗｅｇｇｅｒ峰组（ＷＰＧ）侵入岩。大多数中基性熔岩为斑状结构，斜长石和普通辉石是岩石中最主要的斑晶相。安山岩中尚含有少量斜方辉石，英安岩质熔岩则为细晶质结构。岩颈相岩石的物质组成与熔岩相似，但斑晶含量往往较少。岩石化学成分表明，所有岩石均富铝，为亚碱性火山岩，属于钙碱性岩石系列。岩石富集大离子亲石元素，轻稀土元素略为富集，稀土元素总含量落在钙碱性岩石的含量范围之内并基本随硅含量的增加而增加。岩石具有很低的８７Ｓｒ／８６Ｓｒ比值（０．７０３４左右）和较高的１４３Ｎｄ／１４４Ｎｄ比值（０．５１２７８－０．５１２８８），εＮｄ平均值在＋６左右。８７Ｓｒ／８６Ｓｒ比值与Ｓｉ、Ｋ、Ｒｂ、１／Ｓｒ等元素呈水平线性关系。在Ｃｅ／Ｙｂ－Ｃｅ和Ｔｈ／Ｔａ－Ｔｈ图解上所有岩石的投影点分布趋势相同，趋势线的斜率＞０。上述特征表明，乔治王岛北海岸第三纪火山岩均由上地幔物质直接生成而未受壳源物质的混染。推测在古太平洋板块向南极板块下俯冲的过程中，上地幔发生的不同程度部分熔融所生成的玄武岩岩浆从深部岩浆囊沿构造薄弱带穿过地壳直接喷出形成  相似文献   

8.
We use teleseismic three-component digital data from the Trabzon, Turkey broadband seismic station TBZ to model the crustal structure by the receiver function method. The station is located at a structural transition from continental northeastern Anatolia to the oceanic Black Sea basin. Rocks in the region are of volcanic origin covered by young sediments. By forward modelling the radial receiver functions, we construct 1-D crustal shear velocity models that include a lower crustal low-velocity zone, indicating a partial melt mechanism which may be the source of surfacing magmatic rocks and regional volcanism. Within the top 5 km, velocities increase sharply from about 1.5 to 3.5 km s−1. Such near-surface low velocities are caused by sedimentation, extending from the Black Sea basin. Velocities at around 20 km depth have mantle-like values (about 4.25 km s−1 ), which easily correlate to magmatic rocks cropping out on the surface. At 25 km depth there is a thin low-velocity layer of about 4.0 km s−1. The average Moho velocity is about 4.6 km s−1, and its depth changes from 32 to 40 km. Arrivals on the tangential components indicate that the Moho discontinuity dips approximately southwards, in agreement with the crustal thickening to the south. We searched for the solution of receiver functions around the regional surface wave group velocity inversion results, which helped alleviate the multiple solution problem frequently encountered in receiver function modelling.
Station TBZ is a recently deployed broadband seismic station, and the aim of this study is to report on the analysis of new receiver function data. The analysis of new data in such a structurally complex region provides constraining starting models for future structural studies in the region.  相似文献   

9.
利文斯顿岛火山岩是南极南设得兰岩浆弧的一部分。对岩石的Ｓｒ、Ｎｄ、Ｐｂ同位素组成以及８７Ｓｒ／８６Ｓｒ与１／Ｓｒ、Ｒｂ、Ｋ和ＳｉＯ２关系的研究表明,利文斯顿岛火山岩具有不同的源区特征。百耳斯半岛和史莱夫角的火山岩、西多斯角的粗玄岩和第三纪的英云闪长岩显示出同成因的特点,和依诺特角第四纪橄榄玄武岩一样,它们均具有原生地幔的同位素组分,源区岩浆可能直接来自上地幔。汉那角和中利文斯顿岛火山岩岩浆则受到地壳成分的混染。不相容元素对Ｌａ／Ｓｍ－Ｌａ和Ｃｅ／Ｙｂ－Ｃｅ的研究进一步显示出,上述不同产地的岩石分别为不同源岩浆不同程度部分熔融的产物。结合岩相学和岩石化学特征的研究,提出百耳斯半岛和史莱夫角的火山岩以及西多斯角的粗玄岩是上地幔发生部分熔融所生成的玄武岩岩浆从深部岩浆囊沿构造薄弱带直接喷出形成的。古生代基底岩系的存在对中利文斯顿岛和汉那角火山岩的成因过程有重要的影响,即来自深部岩浆囊的原始岩浆在上地壳中的浅部岩浆囊受到了壳源物质混染。更新世－现代火山岩的源区岩浆直接自上地幔部分熔融生成,火山作用与晚新生代时该区的拉张构造有关。  相似文献   

10.
To provide guides for exploration of porphyry copper mineralization at a district scale, we examine the spatial association between known porphyry copper deposits and geologic features in Benguet, Philippines. The spatial associations between the porphyry copper deposits and strike-slip fault discontinuities, batholithic pluton margins and porphyry plutons are quantified using weights of evidence modeling. In the training and testing district, the porphyry copper occurrences are associated spatially with strike-slip fault discontinuities, batholithic pluton margins and contacts of porphyry plutons within distances of 3 km, 2.25 km, and 1 km, respectively. In addition, the porphyry plutons are associated spatially with strike-slip fault discontinuities and contacts of batholithic plutons within a distance of 2.25 km and 3 km, respectively. Based on these significant spatial associations, predictive maps are generated to delineate zones favorable for porphyry copper mineralization and zones favorable for emplacement of porphyry plutons in Benguet province, Philippines. Validations of the predictive models demonstrate their efficacy in pointing to zones for subsequent follow-up exploration work.  相似文献   

11.
Gravity changes are presented from a series of field microgravity surveys conducted at Mt Etna between August 1994 and November 1996, a period including the 1995–1996 explosive summit activity. Data were collected along a microgravity network of 69 stations at a monthly to annual sampling rate, depending on each subarray of the network.
  Results show that seasonal changes in water level within the volcano may induce gravity changes of up to 20  μgal on Etna's southern slope, and indicate that significant magma movement occurred within and below Etna's edifice between 1994 and 1996. In particular, between September 1994 and October 1995, a mass increase of 2 × 1010  kg occurred 2000  m beneath the summit craters. Between October 1995 and July 1996 this mass was lost, while another 2 × 1010  kg was injected at about 1000  m  a.s.l. into the 1989 fracture system. From the gravity data alone, it is not possible to distinguish whether the first shallow intrusion (1994–1995) was then injected laterally into the 1989 fracture, or summit activity was fed by the first shallow intrusion, while new magma entered the 1989 fracture system.
  While magma was being redistributed within the volcanic edifice, measurements along an E–W-trending profile on the southern slope of the volcano detected some 1.5 × 1011  kg of magma accumulating 2–3  km below sea level between October 1995 and November 1996.  相似文献   

12.
Submarine magmatism and associated hydrothermal fluid flows has significant feedback influence on the petroleum geology of sedimentary basins. This study uses new seismic profiles and multibeam bathymetric data to examine the morphology and internal architecture of post‐seafloor spreading magmatic structures, especially volcanoes of the Xisha uplift, in extensive detail. We discover for the first time hydrothermal systems derived from magmatism in the northwestern South China Sea. Numerous solitary volcanoes and volcanic groups occur in the Xisha uplift and produce distinct seismic reflections together with plutons. Sills and other localized amplitude anomalies were fed by extrusions/intrusions and associated fluid flow through fractures and sedimentary layers that may act as conduits for magma and fluid flows transport. Hydrothermal structures such as pipes and pockmarks mainly occur in the proximity of volcanoes or accompany volcanic groups. Pipes, pockmarks and localized amplitude anomalies mainly constitute the magmatic hydrothermal systems, which are probably driven by post‐seafloor spreading volcanoes/plutons. The hydrothermal fluid flows released by magma degassing or/and related boiling of pore fluids/metamorphic dehydration reactions in sediments produced local overpressures, which drove upward flow of fluid or horizontal flow into the sediments or even seafloor. Results show that post‐seafloor spreading magmatic activity is more intense during a 5.5 Ma event than one in 2.6 Ma, whereas the hydrothermal activities are more active during 2.6 Ma than in 5.5 Ma. Our analysis indicates that post‐seafloor spreading magmatism may have a significant effect on hydrocarbon maturation and gas hydrate formation in the Xisha uplift and adjacent petroliferous basins. Consequently the study presented here improves our understanding of hydrocarbon exploration in the northwestern South China Sea.  相似文献   

13.
Geologic evidence across orogenic plateau margins enables the discrimination of the relative contributions of orogenic, epeirogenic and/or climatic processes that lead to growth and maintenance of those plateaus and their margins. Here, we discuss the mode of formation of the southern margin of the Central Anatolian Plateau (SCAP) and evaluate its time of formation using fieldwork in the onshore and seismic reflection data in the offshore. In the onshore, uplifted Miocene rocks in a dip‐slope topography show monocline flexure over >100 km, km‐scale asymmetric folds verging south, and outcrop‐scale syn‐sedimentary reverse faults. On the Turkish shelf, vertical faults transect the basal latest Messinian of a 10 km fold where on‐structure syntectonic wedges and synsedimentary unconformities indicate pre‐Pliocene uplift and erosion, followed by Pliocene and younger deformation. Collectively, Miocene rocks delineate a flexural monocline at plateau margin scale that is expressed along our on‐offshore sections as a kink‐band fold with a steep flank 20–25 km long. In these reconstructed sections, we estimate a relative vertical displacement of 3.8 km at rates of ca. 0.5 mm/y, and horizontal shortening values <1 %. We use this evidence together with our observations of shortening at outcrop, basin, plateau‐margin and forearc‐system scales to infer that the SCAP forms as a monoclinal flexure to accommodate deep‐seated thickening and shortening since >5 Ma, and to contextualize the plateau margin as the forearc high of the Cyprus subduction system.  相似文献   

14.
We propose and test a conceptual framework for evaluating the relative timing of different types of sedimentary indicators of tectonism in alluvial foreland basin settings. We take the first occurrence of a detrital grain from a newly exposed source‐area lithology to provide the best indicator of the onset of tectonic uplift in the source area. Source‐area unroofing may lag behind initial uplift because of the type, thickness and structure of rocks in the uplifted mountain block, drainage patterns and climate. However, once exposed, advective transport disperses grains quickly throughout fluvial systems. Because of increased subsidence rate from thrust belt loading, an increase in sedimentation rate begins coincident with tectonic load emplacement within the flexural half‐width of the basin. However, farther out into the basin increased sedimentation rates lag behind the composition signal because of time lags associated with propagation of the thrust load and attendant sediment loads into the basin. The progradation of syntectonic gravel lags behind all of these signals as a direct function of the relative proportion of gravel fraction within transported sediment and rates and geometry of subsidence, which selectively traps the coarsest grain‐size fractions in the most proximal parts of the basin. We demonstrate this signal attenuation in the syntectonic Horta–Gandesa alluvial system (late Eocene–Oligocene), exposed along the southeast margin of the Ebro Basin, Spain. The results demonstrate that: (1) the time spans between the compositional signal and the progradation of the gravel front can be geologically significant, on the order of more than a million years within as little as 20 km of the thrust front; and (2) time lags between the signals increase with distance away from the deformation front. No lag time was observed between the first appearance of a new clast composition and the arrival of gravel front when the thrust front was within a few tens of metres from the depositional site. In contrast, the time lag was 0.5–1 Myr when the thrust front was about 5–6 km away and it increased to >1 Myr when the deformation front was about 8 km away. At the most extreme position, when the thrust front was 15–20 km away, the gravel front never reached the study area.  相似文献   

15.
Summary. Results from eight seismic refraction lines, 35–90 km long, in the Bristol Channel area are presented. The data, mostly land recordings of marine shots, have been interpreted mainly by ray-tracing and time-term modelling. Upper layer velocities through Palaeozoic rocks usually fall within the range 4.8–5.2 km s−1. Below the Carboniferous Limestone with a normal velocity of 5.1–5.2 kms−1, the Old Red Sandstone with a velocity of 4.7–4.8 kms−1 acts as a low velocity layer, as do parts of the underlying Lower Palaeozoic succession. In the central South Wales/Bristol Channel area and the Mendips, a 5.4–5.5 km s−1 refractor is correlated with a horizon at or near the top of the Lower Palaeozoic succession. Under the whole area, except for north Devon, a 6.0–6.2 km s−1 basal refractor has been located and is correlated with Precambrian crystalline basement rocks. In general, this refractor deepens southwards from a series of basement highs, which existed before the major movements of the Variscan orogeny in South Wales, resulting in a southerly thickening of the pre Upper Carboniferous supra-basement sequence. In north Devon, a 6.2 km s−1 refractor at shallow depth, interpreted as a horizon in the Devonian or Lower Palaeozoic succession, overlies a deep reflector that may represent the Precambrian crystalline basement.  相似文献   

16.
Janecke  McIntosh  & Good 《Basin Research》1999,11(2):143-165
We examine the basin geometry and sedimentary patterns in the Muddy Creek half graben of south-west Montana by integrating geological mapping, structural and basin analysis, 40Ar/39Ar geochronology, biostratigraphy and reflection seismic data. The half graben formed in late Middle Eocene to early Oligocene (?) time at the breakaway of a regional, WSW-dipping detachment system. Although the structure of the half graben is that of a supradetachment basin, facies patterns and basin architecture do not conform to a recent model for extensional basins above detachment faults. The border fault, the Muddy Creek fault system, consists of three en echelon, left-stepping normal faults separated by two relay ramps. The fault steepens southward toward each en echelon step, ranges in dip from 8 to 60° near the surface, but flattens at depths between 0 and 3 km. A broad ENE-plunging displacement-gradient syncline defines the central part of the half graben and is flanked by narrow SE-and NE-plunging anticlines to the north and south. Fine-grained deposits of the syntectonic basin-fill are thickest in the central syncline and interfinger with footwall-derived conglomerate near the adjacent anticlines. These facies patterns suggest that folding was coeval with extension and sedimentation in the half graben. Pre-extensional volcanic rocks and interbedded conglomerate filled a major ESE-trending palaeovalley along the future axis of the Muddy Creek half graben. Synextensional sedimentary deposits include lacustrine and paludal shale, mudstone and sandstone ponded in the centre of the half graben, and a narrow (typically <1.5 km wide) fringe of coarse alluvial-fan and fan-delta conglomerate and sandstone derived from the footwall. Angular unconformities and rock-slide deposits occur only locally within the syntectonic sequence. These facies patterns agree well with the half-graben depositional model of Leeder & Gawthorpe but not with a more recent supradetachment basin model of Friedmann & Burbank despite the demonstrably low dip-angle of the basin-bounding normal fault. These data show that it may not be possible to differentiate between supradetachment basins and half graben with steeper border faults using the architecture of the associated basin-fill deposits.  相似文献   

17.
Aquifer-bearing intermontane sedimentary basins of the Sierras Pampeanas in the northwest of Argentina are in general very deep and narrow and contain economically important deposits of Tertiary sediments. This paper presents the results of a study to characterize the sedimentary basin bounded to the west by Sierra de Famatina and to the east by Sierra de Velasco, where an electromagnetic sensing technique, the magnetotelluric (MT) method, was applied. 12 MT sites were deployed along a 30  km E–W transect. Some of the data collected were used to derive a 2-D resistivity model of the basin. The model shows a subsurface trough with a thick (approximately 8  km) sedimentary sequence above it. Anomalous behaviour of the E–W electric-field component ( E y ) was detected in the period range 1–100  s, where the amplitude of this component was below the instrumental noise level. The cause of this anomaly is not known, but it might be due to the presence of an embedded conducting body between 8 and 10  km, which would give rise to N–S current channelling.  相似文献   

18.
A magnetotelluric study of the Alpine Fault, New Zealand   总被引：1，自引：0，他引：1  
Magnetotelluric soundings have been made at seven locations on a 4  km profile crossing the Alpine Fault in the South Island of New Zealand. The 'distortion' techniques of Groom & Bailey (1989 ) and Lilley (1998a , b ) have been used to derive regional apparent resistivity and phase curves that correspond to electromagnetic induction in orientations parallel and perpendicular to the fault. 2-D inversion of the regional responses reveals that a narrow (<1  km wide) conductive zone is associated with the Alpine Fault. This conductor is most probably related to the heating of deep circulating meteoric water in a region in which enhanced temperatures occur at shallow depth due to the tectonic uplift of the Southern Alps.  相似文献   

19.
The Stara Planina is an E–W-trending range within the Balkan belt in central Bulgaria. This topographically high mountain range was the site of Mesozoic through early Cenozoic thrusting and convergence, and its high topography is generally thought to have resulted from crustal shortening associated with those events. However, uplift of this belt appears to be much younger than the age of thrusting and correlates instead with the age of Pliocene–Quaternary normal faulting along the southern side of the range. Flexural modelling indicates the morphology of the range is consistent with flexural uplift of footwall rocks during Pliocene–Quaternary displacement on S-dipping normal faults bounding the south side of the mountains, provided that the effective elastic plate thickness of 12  km under the Moesian platform is reduced to about 3  km under the Stara Planina. This small value of elastic plate thickness under the Stara Planina is similar to values observed in the Basin and Range Province of the western United States, and suggests that weakening of the lithosphere is due to heating of the lithosphere during extension, perhaps to the point that large-scale flow of material is possible within the lower crust. Because weakening is observed to affect the Moesian lithosphere for ≈10  km beyond (north of) the surface expression of extension, this study suggests that processes within the uppermost mantle, such as convection, play an active role in the extension process. The results of this study also suggest that much of the topographic relief in thrust belts where convergence is accompanied by coeval extension in the upper plate (or 'back arc'), such as in the Apennines, may be a flexural response to unloading during normal faulting, rather than a direct response to crustal shortening in the thrust belt.  相似文献   

20.
The Canary Islands swell: a coherence analysis of bathymetry and gravity   总被引：2，自引：0，他引：2  
The Canary Archipelago is an intraplate volcanic chain, located near the West African continental margin, emplaced on old oceanic lithosphere of Jurassic age, with an extended volcanic activity since Middle Miocene. The adjacent seafloor does not show the broad oceanic swell usually observed in hotspot-generated oceanic islands. However, the observation of a noticeable depth anomaly in the basement west of the Canaries might indicate that the swell is masked by a thick sedimentary cover and the influence of the Canarian volcanism. We use a spectral approach, based on coherence analysis, to determine the swell and its compensation mechanism. The coherence between gravity and topography indicates that the swell is caused by a subsurface load correlated with the surface volcanic load. The residual gravity/geoid anomaly indicates that the subsurface load extends 600 km SSW and 800 km N and NNE of the islands. We used computed depth anomalies from available deep seismic profiles to constrain the extent and amplitude of the basement uplift caused by a relatively low-density anomaly within the lithospheric mantle, and coherence analysis to constrain the elastic thickness of the lithosphere ( Te ) and the compensation depth of the swell. Depth anomalies and coherence are well simulated with Te =28–36 km, compensation depth of 40–65 km, and a negative density contrast within the lithosphere of ∼33 kg m−3. The density contrast corresponds to a temperature increment of ∼325°C, which we interpret to be partially maintained by a low-viscosity convective layer in the lowermost lithosphere, and which probably involves the shallower parts of the asthenosphere. This interpretation does not require a significant rejuvenation of the mechanical properties of the lithosphere.  相似文献   

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